Unimodal relationship between small-scale barnacle recruitment and the density of pre-existing barnacle

7 Recruitment is a key demographic process for population persistence. This paper focuses on 8 barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. 9 Lawrence coast of Nova Scotia (Canada), ice scour is common during the winter. At the onset of 10 intertidal barnacle recruitment in early May (after sea ice has fully melted), mostly only adult 11 barnacles and bare substrate are visible at high elevations in wave-exposed habitats. We 12 conducted a multiannual study to investigate if small-scale barnacle recruitment could be 13 predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult 14 density range (ca. 0–130 individuals dm -2 ), the relationship between adult density and recruit 15 density (scaled to the available area for recruitment, which excluded adult barnacles) was 16 unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm -2 ), the 17 relationship between adult and recruit density was positive and resembled the lower half of the 18 unimodal relationship. Overall, adult barnacle density was able to explain 26–40% of the 19 observed variation in recruit density. The unimodal adult–recruit relationship is consistent with 20 previously documented intraspecific interactions. Between low and intermediate adult densities, 21 the positive nature of the relationship relates to the previously documented fact that adult 22 barnacles attract settlement-seeking larvae, which likely enhances local population persistence 23 where it is most needed. Between intermediate and high adult densities, where population 24 persistence may be less compromised and the abundant adults may limit recruit growth and 25 survival, the negative nature of the relationship suggests that adult barnacles at increasingly high 26 densities stimulate larvae to settle elsewhere. The unimodal pattern may be especially common 27 on shores with moderate rates of larval supply to the shore, because high rates of larval supply 28 may swamp the coast with settlers, decoupling recruit density from local adult abundance. 29 PeerJ reviewing PDF | (2017:04:17214:1:1:CHECK 16 May 2017) Manuscript to be reviewed